This is the example of port 993 (imaps) for imap4.example.com server to check the expiring date of the cert.pem (etc)

%  openssl x509 -text -noout -in <(openssl s_client -connect imap4.example.com:993 )  | grep -A3 Validity
depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
verify return:1
depth=0 CN = imap4.example.com
verify return:1
        Validity
            Not Before: Oct  9 12:42:44 2019 GMT
            Not After : Jan  7 12:42:44 2020 GMT
        Subject: CN = imap4.example.com